1. Field of the Invention
The present invention relates to an image forming process by image intensification, in particular, to an image forming process which restrains the formation of fog in an intensification step, restrains the influence of intensification checking materials such as I.sup.- or Br.sup.- ions, and greatly improves the intensification effect. Even more specifically, the invention relates to an image forming process by intensification utilizing a photographic element having image forming unit layers each containing a sufficient amount of a color former having associated therewith a photosensitive silver halide in an amount less than the stoichiometrical amount thereof based on the amount of the color former.
2. Description of the Prior Art
Various processes have hitherto been known for intensifying the image formation of silver halide photographic materials by subjecting the photographic materials to development and intensification processings.
The formation of a dye by the oxidation of a p-phenylenediamine color developing agent caused by the decomposition of hydrogen peroxide on the surface of silver catalyst in the presence of a color former and the subsequent coupling of the oxidation product with the color former (this phenomenon is called "color intensification") is described in, for instance, Friedman; "History of Color Photography", 2nd Ed., 406(1956).
Furthermore, other various photographic processes wherein the decomposition of peroxides on the surfaces of noble metals is utilized are described in German Offenlegungusschrift Nos. 1,813,920; 1,950,102; 1,955,901; 1,961,029; 2,044,833; 2,044,993; 2,056,360; 2,056,359; and 2,120,091. Moreoever, it has long been known from old that peroxides are decomposed on the surfaces of noble metals as described in, for example, "Shokubai Kogaku Koza (Catalyst Industry)"; Vol. 2, published in 2966 by Chijin Shokan K.K.
On the other hand, color intensification processes by cobalt complex salts at the surfaces of noble metals are described in, for example, Japanese Patent Applications (Laid Open) Nos. 9728/'73; 9729/'73; 48130/'73; 84229/'74; 84239/'74; 84240/'74; 97614/'74; 102340/'74 and 102341/'74.
Also, color intensification processes by halogenous acid salts such as chlorites, etc., are described in, for example, Japanese Patent Applictions Nos. 128327/'74 and 139917/'74.
Compounds possessing an intensifying action, such as the above-indicated peroxides, halogenous acid salts, and cobalt(III) complex salts are called "intensifying agents" and a processing bath containing the intensifying agent is called an "intensifying bath" or "intensifier".
In the field of color photographic processing to which the process of this invention belongs, a color photographic material prepared by coating a support with silver halide emulsions containing cyan, yellow, and magneta color formers is image-wise exposed and then subjected to a series of photographic procedures for reproducing color images on the photographic material.
The fundamental steps of the processing are a color development step and a silver removal step. That is, in the color development step, the exposed silver halides in the silver halide color photographic material are reduced by a color developing agent to form silver images, and, at the same time, the color developing agent is oxidized. The oxidized color developing agent reacts with the color formers in the color photographic material to form dye images. Thereafter, the color photographic material is processed in the silver removal step to oxidize the silver formed in the previous step by the action of an oxidizing agent (called "a bleaching agent"). The color photographic material is then processed in a fix step, that is, the oxidized silver is dissolved by a complexing agent for silver ions (called a "fixing agent") and removed from the photographic material. Thus, the color photographic material processed in these steps has dye images only.
Generally, the development procedure further includes auxiliary steps for improving the photographic and physical qualities of the dye images formed and to improve the preservability of the color images in addition to the two fundamental steps of color development and silver removal. For example, as these auxiliary steps, there are illustrated the steps of hardening for preventing the photosensitive layers or films of the photographic material from being excessively softened during processing, stopping for effectively stopping the development reaction, image stabilization to stabilize the dye images formed, and a film removal bath for removing a backing layer formed on the support of the photographic material.
This kind of color photographic process as described above has been employed on a world wide basis in the color photographic field since 1940.
In the above-mentioned color intensification process, it is necessary to quickly carry out the oxidation of a reducing agent such as a color developing agent on the surface of a small amount of a metallic silver catalyst comprising developing nuclei of silver halide grains, and developing silver to obtain a dye density the same as in these conventional processes. For this purpose, it is desired to increase the activity of a processing bath containing the intensifying agent or to employ a reducing agent such as a color developing agent having a higher activity than conventional ones as is described in, for example, Japanese Patent Applications (Laid Open) 11534/'72 and 64932/'73.
When such a highly active intensification process is used, a photographic material using silver halide as the photosensitive material undergoes coloring even in unexposed portions in the intensification bath to form fog (referred to as color intensification fog), and this is a great problem in the case of the practical use of intensification processes.
While it might be concluded that the occurence of fog in the intensification step could be prevented by incorporating an antifoggant in the intensification bath, if an anti-foggant such as potassium bromide or sodium bromide is incorporated in the intensification bath, the intensifying action of the bath is also greatly suppressed. Further, the use of an organic anti-foggant possessing high adsorptive power (i.e., a compound having a mercapto group) greatly suppresses the intensifying power of the bath. On the other hand, an organic antifoggant possessing a weak adsorptive power (e.g., 6-nitrobenzimidazole) may be effectively used for this purpose, but if the amount of the compound is over 1 g/liter, the intensifying power of the bath is greatly suppressed.
Furthermore, an intensification bath containing a peroxide or halogenous acid salt as an intensifying agent is greatly degraded in intensifying activity by the presence of potassium bromide added to the developer as an antifoggant or by bromine ions or iodine ions released during the development of silver halide grains.